In fill level sensors operating according to the FMCW or pulse-transit time method, electromagnetic or acoustic waves are emitted in the direction of a feed material surface. Following this, a sensor records the echo signals reflected by the feed material, by the objects built into the container, and by the container itself, and from this derives the position of a surface of at least one of the feed materials contained in the container.
With the use of acoustic or optical waves the signal generated by the fill-level measuring device generally propagates freely in the direction of the feed material surface to be measured. In devices that use radar waves for measuring the feed material surface, both free propagation in the direction of the medium to be measured can be considered, and propagation in the interior of a waveguide that guides the radar waves from the fill-level measuring device to the medium. In devices operating according to the principle of the guided microwave the high-frequency signals are guided along a waveguide to the medium.
At the surface of the medium or fill level to be measured, some of the arriving signals are reflected and after a corresponding transit time return to the fill-level measuring device. The non-reflected signal components penetrate the medium and in the medium continue to propagate, corresponding to the physical characteristics of the medium, in the direction of the container bottom. At the container bottom these signals, too, are reflected and after passing through the medium and the overlaid atmosphere return to the fill-level measuring device.
The fill-level measuring device receives the signals, which have been reflected at different positions, and from them determines the distance to the feed material according to known methods. The determined distance to the feed material is made available externally. Such provision can be implemented in an analog form (4 . . . 20 mA interface) or in a digital form (fieldbus).
All the methods share a common feature in that on its way from the fill-level measuring device to the feed material surface the signal used for measuring is normally in the region of influence of a further medium, which hereinafter is referred to as the overlay medium. This overlay medium is situated between the fill-level measuring device and the surface of the medium to be measured, and is generally represented by a liquid or by a gaseous atmosphere.
In a predominant number of applications there is air above the medium to be measured. Since the propagation of electromagnetic waves in air differs only insignificantly from that in a vacuum, there is no need to carry out any special corrections of the signals that are reflected, through the air back to the fill-level measuring device, by the feed material, by the objects built into the container, and by the container itself.
Furthermore, however, in process containers of the chemical industry many types of chemical gases and gas mixtures can occur as overlay media. Depending on the physical characteristics of these gases or gas mixtures, the propagation characteristics of electromagnetic waves are changed when compared to propagation in a vacuum or in air.
Known attempts at determining media characteristics and container characteristics are often associated with significant weaknesses.